1. Field of the Invention
This application relates generally to electrodes in electrohydrodynamic or electrostatic devices such as electrohydrodynamic fluid accelerators and electrostatic precipitators, and particularly to classes of materials that can be used to form such electrodes.
2. Description of the Related Art
Many electronic devices and mechanically operated devices require air flow to help cool certain operating systems by convection. Cooling helps prevent device overheating and improves long term reliability. It is known to provide cooling air flow with the use of fans or other similar moving mechanical devices; however, such devices generally have limited operating lifetimes, produce noise or vibration, consume power or suffer from other design problems.
The use of an ion flow air mover device, such as an electrohydrodynamic (EHD) device or electro-fluid dynamic (EFD) device, may result in improved cooling efficiency, reduced vibrations, power consumption, electronic device temperatures, and noise generation. This may reduce overall device lifetime costs, device size or volume, and may improve electronic device performance or user experience.
Devices built using the principle of the ionic movement of a fluid are variously referred to in the literature as ionic wind machines, electric wind machines, corona wind pumps, electro-fluid-dynamics (EFD) devices, electrohydrodynamic (EHD) thrusters and EHD gas pumps. Some aspects of the technology have also been exploited in devices referred to as electrostatic air cleaners or electrostatic precipitators.
In general, EHD technology uses ion flow principles to move fluids (e.g., air molecules). Basic principles of EHD fluid flow are reasonably well understood by persons of skill in the art. Accordingly, a brief illustration of ion flow using corona discharge principles in a simple two electrode system sets the stage for the more detailed description that follows.
With reference to the illustration in FIG. 1, EHD principles include applying a high intensity electric field between a first electrode 10 (often termed the “corona electrode,” the “corona discharge electrode,” the “emitter electrode” or just the “emitter”) and a second electrode 12. Fluid molecules, such as surrounding air molecules, near the emitter discharge region 11 become ionized and form a stream 14 of ions 16 that accelerate toward second electrode 12, colliding with neutral fluid molecules 22. During these collisions, momentum is imparted from the stream 14 of ions 16 to the neutral fluid molecules 22, inducing a corresponding movement of fluid molecules 22 in a desired fluid flow direction, denoted by arrow 13, toward second electrode 12. Second electrode 12 may be variously referred to as the “accelerating,” “attracting,” “target” or “collector” electrode. While stream 14 of ions 16 is attracted to, and generally neutralized by, second electrode 12, neutral fluid molecules 22 continue past second electrode 12 at a certain velocity. The movement of fluid produced by EHD principles has been variously referred to as “electric,” “corona” or “ionic” wind and has been defined as the movement of gas induced by the movement of ions from the vicinity of a high voltage discharge electrode 10.
Ozone (O3), while naturally occurring, can also be produced during operation of various electronics devices, including EHD devices, photocopiers, laser printers and electrostatic air cleaners, and by certain kinds of electric motors and generators, etc. Elevated ozone levels have been associated with respiratory irritation and certain health issues. Therefore, ozone emission can be subject to regulatory limits such as those set by the Underwriters Laboratories (UL) or the Environmental Protection Agency (EPA). Accordingly, techniques to reduce ozone concentrations have been developed and deployed to catalytically or reactively break down ozone (O3) into the more stable diatomic molecular form (O2) of oxygen.
Some of the characteristics in which known emitter and collector materials are often deficient entail surface chemistry and catalysis. For example, EHD device performance reduction or failure can be caused by gradual coating of the emitter with silica. Still other EHD devices produce unacceptable concentrations of ozone in the air transported through the device. Additionally, some electrodes may be susceptible to oxidation, corona erosion, or accumulation of detrimental materials. The term “corona erosion” refers to various adverse effects from a plasma discharge environment including enhanced oxidation, and etching or sputter of emitter surfaces. In general, corona erosion can result from any plasma or ion discharge including, silent discharge, AC discharge, dielectric barrier discharge (“DBD”) or the like.
Generally, many desirable electrode materials properties can be achieved by forming the emitter and collector being made of particular metals. For example, the emitter may be made of tungsten and the collector made of aluminum to provide desired conductivity, hardness and strength. However, pure metals are often deficient in some regard with respect to other desirable materials characteristics. One proposed solution is to use an alloy in place of a pure metal. While various metals or alloys may be selected to address a particular one of these performance parameters, a combination of two materials having known performance characteristics often yields an alloy or compound exhibiting significantly different characteristics.
For example, a collector may be made of an aluminum alloy to increase its hardness. Similarly, the emitter electrode may be made of stainless steel, so that the three elements of iron, nickel and chromium are present and exposed to the atmosphere in which the EHD device operates. While each of the elements present in the alloy will contribute in some way to the overall characteristics required, alloys of such metals do not always provide the same desirable characteristics as the pure metals would alone and such compound characteristics are not always readily predictable.
Many metal alloys exhibit duplex or higher ordered microstructures. For example, mixing of lead with tin results not in a mixture of pure lead and tin, but a two-phase mixture consisting of lead containing tin and tin containing lead. The alloy no longer contains either pure lead or pure tin so the beneficial effects of these elements may be altered, diminished or lost. While some phases formed on alloying may present other beneficial materials characteristics, such beneficial properties are not readily determinable or predictable without extensive testing as the new phases do not present the same properties as the pure components.
Accordingly, improvements are sought in enhancing electrode performance by providing predictable combined performance characteristics through combination of selected materials.